In recent years, polymethine compounds have been in wide use, among others, as light-to-heat converting agents for optical recording media, for near-infrared absorbing filters or plate-making materials for which laser beams are to be utilized. These polymethine compounds generally form a salt structure with a counter ion, and researches have been made concerning polymethine compounds improved in various ways with respect to the counter ion for the purpose of improving the solubility in solvents, the compatibility with resins, the durability and the sensitivity to laser beams.
For producing polymethine compounds containing a desired counter ion species, a method is known which comprises once synthesizing a polymethine compound containing a counter ion species relatively easy to synthesize, for example a perchlorate ion, tetrafluoroborate ion or p-toluenesulfonate ion, dissolving the polymethine compound obtained and a compound containing the desired counter ion species in a solvent, for example dimethylformamide (hereinafter referred to as “DMF”) to cause counter ion species interchange in the solvent, as described in Example 1 in Japanese Kokai Publication 2000-302992, for instance.
Further, as is described in Example 1 in Japanese Kokai Publication H11-1626, a synthetic method is known which comprises once synthesizing a polymethine compound containing a counter ion species relatively easy to synthesize, for example a perchlorate ion, tetrafluoroborate ion or p-toluenesulfonate ion, reacting the polymethine compound obtained with an alkali such as caustic soda to give an intermediate compound (hereinafter referred to as “hydroxy compound”) resulting from elimination of the counter ion and having a structure represented by the formula (B) given below, and further reacting this hydroxy compound with a compound containing the desired counter ion.
(In the formula (B), R1, R2, R3, L and X are as defined later herein referring to the formula (I).)
However, as regards the former method, the range of producible counter ion species is limited and, further, the counter ion species exchange is incomplete and it is therefore difficult to obtain, by that method, the high-purity compounds containing a desired counter ion species in high yields. As for the latter method, on the other hand, the hydroxy compounds are very unstable and, therefore, this method of producing polymethine compounds using those hydroxy compounds is not suitable as an industrial production method since the purity and yield of the products are low and a complicated purification process is required for obtaining high-purity products.
As a compound structurally close to the polymethine ether compounds of the invention, there is the compound (A) having the structure shown below as described in Dyes and Pigments, 46 (2000), 164. However, there is no description about the use thereof, among others. If the compound (A) is used to produce the corresponding polymethine compound, the polymethine compound obtained will show an absorption wavelength range fairly longer (≧1000 nm) than the general-purpose semiconductor laser wavelength range and, further, the raw materials for the production thereof are special and the production cost is increased accordingly, hence the industrial use value will be restricted.
